Magnetic sensor devices can be used in a variety of fields including communications, defense, magnetic field sensors, medical, transportation, homeland security, computers, manufacturing, mineral exploration, underground infrastructure, and transportation. Depending upon the application, there are various requirements, factors, and tradeoffs such sensitivity, cost, and power consumption that must be considered for the various applications. For example, for automotive and high volume applications the prime factor is cost. In neuromagnetism the prime factor is sensitivity. For use in unattended ground sensor networks the prime factors are cost, sensitivity, power consumption, and visibility. For Department of Defense purposes, the Army requires magnetic sensors that can be used in unattended ground sensors (UGS) networks, at choke points, for mine detection, and general situational awareness. The Navy has applications in areas such as anti-submarine warfare (ASW), mine detection, harbor control, and checking demagnetization of ships. The Air Force has related uses.
Further examples of uses of magnetic detection devices include identification and financial credit cards, Government facilities protection, airport security, border control, drug control & enforcement, tunnel & underground facilities detection, navigation equipment, traffic light control, and ensuring airplane doors are securely closed (detection).
Illustrated in FIG. 8A is a chart showing the Tesla sensitivities required for various potential applications; e.g., drug delivery and cardiomagnetism, which requires sensitivity on the order of 10 pT (pico Tesla) and is presently done with SQUIDS. Another application is neuromagnetism which operates around 100 fT (10−15) and is presently done with SQUIDS.
As to the types of magnetometers, as illustrated in FIG. 8B, scalar magnetometers measure the total strength (or total field), i.e., the magnitude of the magnetic field to which they are subjected, and vector magnetometers have the capability to measure the vector component of the magnetic field in a particular direction. The use of three orthogonal vector magnetometers allows the magnetic field strength, inclination and declination to be uniquely defined. Examples of vector magnetometers are fluxgates, and superconducting quantum interference devices (SQUIDs). Other examples of vector sensors are coil based, magnetoresistive, and hall effect sensors.
FIG. 8C shows the approximate Tesla ranges for the basic kinds of magnetic sensors, vector and total field magnetometers, with the frequency range plotted against Tesla sensitivity. Total magnetometers use the field dependence of quantum energy levels. Types of total field magnetometers include proton precession magnetometers, Overhauser magnetometers (which increase nuclear polarization via hyperfine interaction to electron's moment), and optically pumped magnetometers that usually use Cs or Rb; uncertainty principal limits precision at high sampling rate.
FIG. 9 is a graphical illustration of magnetic noise from various weapons and vehicles showing the magnetic anomaly signal versus the range in meters.
FIG. 9A graphically represents the 1/f noise versus frequency. Slow-moving battlefield targets generate anomalies in the low frequency, f<1 Hz, high-noise region (represented by shaded area 10S). The 1/f noise limits magnetic anomaly detectors.
FIG. 8B is a graphical illustration of the problems associated with the total field and vector magnetometers.
FIG. 8B shows the relative reference distinctions between total field and vector sensors. The advantages and disadvantages are shown in the following table.
Type of SensorAdvantagesDisadvantagesTotal FieldSensitiveCostly, best at lowSensorsInsensitive tofrequenciesrotationalVibrationsFluxgateSensitiveCostly, large,consumes a lot ofpowerCoil basedSensitive at highInsensitive at lowfrequenciesfrequencies, costly,MagnetoresistiveLow cost, small,large 1/f noisepotentiallysensitive
Vector magnetometers provide more information but are affected by rotational vibrations, which makes them difficult to use on vehicles. Vector magnetometers include SQUID, coil based, Fluxgate and Manetoresistive, and Hall effect. Total field measurements usually does not measure the total field from an object; but generally measures the projection of the total field from an object on earth's total field. Total field magnetic sensors include optically pumped, Overhauser, proton precession magnetometers and Nuclear precession. Total field magnetometers employ the field dependence of quantum energy levels.
Examples of patented MEMS devices include U.S. Pat. No. 6,501,268 to Edelstein, et al., entitled “MAGNETIC SENSOR WITH MODULATING FLUX CONCENTRATOR FOR 1/F NOISE REDUCTION,” U.S. Pat. No. 6,670,809 to Edelstein, et al., also entitled “MAGNETIC SENSOR WITH MODULATING FLUX CONCENTRATOR FOR 1/F NOISE REDUCTION,” and U.S. Pat. No. 7,195,945, to Edelstein, et al., entitled “MINIMIZING THE EFFECT OF 1/F NOISE WITH A MEMS FLUX CONCENTRATOR,” all three of which are hereby incorporated by reference as though fully rewritten herein. An example of a publication disclosing a MEMS device is found in an article entitled “Hybrid Magnetoresistive/microelectromechanical Devices for Static Field Modulation and Sensor 1/f Noise Cancellation,” by A. Guedes et al., Journal of Applied Physics 103, 07E924 (2008). The Guedes article reported that low frequency 1/f noise in magnetoresistive MR spin valve sensors was suppressed by modulating an external dc magnetic field at high frequency microelectromechanical system using a MEMS microcantilever structure with an integrated magnetic flux guide. With this hybrid MR/MEMS device, direct detection of dc magnetic fields in the sensor high frequency thermal noise regime was reportedly achieved. The microcantilever was actuated using a gate electrode by applying an ac voltage with frequency f, causing it to oscillate at 2f. Measurements were reported to show detection of a dc magnetic field at 2f frequency (400 kHz), where sensor 1/f noise is two orders of magnitude lower than dc.
There has been a focus on magnetoresistive sensor technology because of the opportunity for producing low cost magnetic sensors. Magnetic sensors or transducers are generally passive sensors with desirable attributes for several types of applications that include insensitivity to weather conditions, the requirement of only a small bandwidth, and the unique ability to “see through” walls and foliage without attenuation. Furthermore, in military applications it is generally difficult to make a weapon or vehicle that does not include ferrous material that can be detected by magnetic sensors. Though the permanent magnetic moment of the ferrous material can be minimized by “deperming,” which is a process of reduction of permanent magnetism, the distortion of the earth's magnetic field due to the magnetic permeability is typically difficult to hide. Data from magnetic sensors can be combined with the data from other sensor modalities such as acoustic and seismic sensors to characterize or identify and track targets. Specifically, in military applications magnetic sensors can be used for perimeter defense, at check points, as part of a suite of sensors in unattended ground sensor networks, and on unmanned ground vehicles (UGVs) and unmanned air vehicles (UAVs). Moreover, magnetic sensors or transducers can also be employed to monitor rooms and passageways that have been cleared by military personnel.
The magnetic signals from military targets come from the internal motion of ferromagnetic parts, electrical currents, and the motion of targets containing ferromagnetic material relative to the magnetic sensor. The latter can arise either from the motion of the target or the sensor. All of these magnetic signals often occur at low frequencies, typically less than 100 Hz. Because the earth's field is usually larger than the field generated by the target, it is generally difficult to detect the magnetic signals that occur at low frequencies, typically less than 100 Hz. Moreover, it is generally difficult to detect the magnetic target without having the field change by relative motion between the target and the sensor. At low frequencies the electric and magnetic fields are decoupled. The magnetic strength from a target at a distance greater than the target size is usually like that of a magnet dipole. Because of the relatively short detection range of magnetic sensors, a large number of magnetic sensors may have to be used if one wants to guarantee detection over a large area.
Magnetoresistance sensors are candidate low cost sensors because they can be mass produced by batch processing on silicon wafers and the drive and read out electronics are relatively simple. The resistance of a magnetoresistance sensor is sensitive to the magnitude and direction of the magnetic field. The types of magnetoresistive sensors include anisotropic magnetoresistance (AMR) sensors, giant magnetoresistance (GMR) sensors, and magnetic tunneling junction (MTJ) sensors. Magnetoresistance values as large as 220% may be observed in CoFe(100)/MgO(100)/CoFe(100) MTJ sensors at room temperature.
To detect the relative motion between the target and the magnetic sensor generally requires high sensitivity in the frequency range f≦1 Hz. Unfortunately, most magnetoresistance sensors tend to suffer from 1/f noise. Furthermore, there is generally a tendency for the sensors that have a larger response to magnetic fields to also have more 1/f noise. Thus, 1/f noise is a significant problem in applying magnetic sensors to military applications.
Another problem in using magnetoresistive sensors at low frequencies and at low fields is that the induced percentage change in the resistance is generally small. Thus, with a single device one must accurately measure a small change in a large signal. Because of this problem, most magnetoresistive sensors have several sensors that are arranged in bridge circuits to eliminate the DC bias offset.
Anisotropic magnetoresistance (AMR) sensors are probably the most sensitive, commercial, magnetoresistance sensors to use at frequencies of 1 Hz or less. This is true despite the fact that their magnetoresistance, approximately 4%, is relatively small. The reason for this is that AMR sensors have less 1/f noise than other magnetoresistive sensors. However, there remains a need for a device that can eliminate the problem of 1/f noise in small magnetic sensors.
When measuring magnetic flux, it is generally advisable to increase the input dimensions of the concentrator in order to provide for greater detection capability. In flux concentrators which move or oscillate, increasing the dimensions generally translates to an increase in mass. For example, U.S. Pat. No. 6,670,809 describes an approach that, improves the sensitivity of magnetic sensors in general that operate at low frequencies by using flux concentrators that modulate an observed sensed magnetic field with low frequency signals, thereby shifting this observed field to higher frequencies where the 1/f noise of the sensor is smaller. This is accomplished by providing a magnetic flux concentrator deposited on suspended microelectromechanical system(MEMS) structure. Such a combined device is used in a magnetometer.
The moving flux concentrators are utilized to periodically increase the sensed magnetic flux density at the position of the sensor 10. A layered material forming part of the flux concentrator 40 can comprise a thick film of a soft ferromagnetic material with a large magnetic permeability.
There are several types of magnetometers (magnetic sensors with external instrumentation) currently used. The least expensive and least sensitive devices have resolution of about 10−1 Oersted (Oe)/Hz1/2 and typically are Hall effect devices. These devices work by sensing a voltage change across a conductor or semiconductor placed in a magnetic field. Such devices do not lend themselves for applications requiring greater sensitivity, such as that required in brain scan devices and magnetic anomaly detection devices. Flux gate magnetometers are more sensitive, having resolution of approximately 10−6 Oe/Hz1/2. Flux gate magnetometers use a magnetic core surrounded by an electromagnetic coil and are difficult to microfabricate. Additionally flux gate magnetometers require relatively large amount of power and accordingly do not lend themselves to low-cost, compact, portable design. The most sensitive magnetometers called SQUIDS (superconducting quantum interference devices) have a resolution of about 10−10 Oe/Hz1/2. However, because they include a superconducting element, these apparatus must include cooling means at liquid gas temperatures, making them extremely bulky and expensive to operate. Their size limits their utility because the active superconducting element cannot be placed directly adjacent to the source of the magnetic field, for example the brain. Accordingly, there is need for small, inexpensive, low power magnetometers that have sufficient sensitivity to be useful for a variety of magnetometer applications at low frequencies. Magnetoresistive sensors are suited for low-cost, medium-sensitivity application.
A well known way of increasing the sensed magnetic field by a magnetic sensor is by use of a flux concentrator, which can enhance a sensed magnetic field by as much as a factor of 50. An example of such a device is taught in U.S. Pat. No. 5,942,895, entitled “Magnetic field sensor and current and/or energy sensor,” that use Hall sensors with flux concentrator components.
Magnetic sensors used to detect objects that move slowly typically possess considerable low frequency 1/f-type noise, an unwanted condition. In general, there is a tendency for such devices that have higher sensitivity to also exhibit higher 1/f-type noise. This generally occurs when using magnetoresistive-type magnetic sensors, as reported in van de Veerdonk et al., “1/f Noise in Anisotropic and Giant Magnetoresistive Elements,” J. Appl. Phys. 82, 6152 (1997).
A magnetic sensor (magnetometer) that addresses 1/f-type noise is taught in U.S. Pat. No. 4,864,237. This disclosure teaches of an apparatus for measuring magnetic fields, that change only at extremely low frequencies. The apparatus uses a SQUID magnetometer that includes a superconducting flux transformer that inductively couples a detected signal into a d-c SQUID sensor. This magnetometer can optionally include a device for modulating the detected signal in a frequency range characteristic of low-noise operation of the SQUID. The modulation frequencies are generally above 1 Hz and optionally even above 1-kHz. Limitations of this device include need for cryogenic operation, which inherently do not lend themselves to relatively lost cost, low power use.
Thus, there is need, for small, low-cost, low-power-consuming magnetic sensors having sensitivities capable of meeting the varied applications listed above for detecting low frequency signals and minimize 1/f-type noise. There exists a need for a magnetic sensor with flux concentrator having sufficient sensitivity for a variety of applications that minimize the effects of 1/f-type noise. There exists a need for a magnetic sensor with flux concentrator that is inexpensive to manufacture, having a magnetic sensor having high sensitivity, yet not having to be concerned with 1/f-type noise associated with that type of sensor. There also exists a need for increased input dimensions of the concentrator in order to provide for greater detection capability.